What Are the Structural Processes for Plastic Parts?

What Are the Structural Processes for Plastic Parts?

The structural process design for plastic parts mainly involves considerations such as geometry, dimensional accuracy, draw ratio, surface roughness, wall thickness, draft angle, hole diameter, fillet radii, mold draft angle, and reinforcement ribs. This article will elaborate on each of these points and discuss how to optimize these elements during the thermoforming process to improve product quality and production efficiency.

1. Geometry and Dimensional Accuracy

Since plastic thermoforming is a secondary processing method, especially in vacuum forming, there is often a gap between the plastic sheet and the mold. Additionally, shrinkage and deformation, especially in protruding areas, can cause wall thickness to become thinner, leading to a decrease in strength. Therefore, plastic parts used in vacuum forming should not have overly stringent requirements for geometry and dimensional accuracy.

During the forming process, the heated plastic sheet is in an unconstrained stretching state, which can lead to sagging. Coupled with significant cooling and shrinkage after demolding, the final dimensions and shape of the product can be unstable due to temperature and environmental changes. For this reason, thermoformed plastic parts are not suitable for precision molding applications.

2. Draw Ratio

The draw ratio, which is the ratio of the part’s height (or depth) to its width (or diameter), largely determines the difficulty of the forming process. The larger the draw ratio, the more difficult the molding process becomes, and the greater the likelihood of undesirable issues such as wrinkling or cracking. Excessive draw ratios significantly reduce the strength and stiffness of the part. Therefore, in actual production, a range below the maximum draw ratio is typically used, usually between 0.5 and 1.

The draw ratio is directly related to the minimum wall thickness of the part. A smaller draw ratio can create thicker walls, suitable for thin sheet forming, while a larger draw ratio requires thicker sheets to ensure that the wall thickness does not become too thin. Additionally, the draw ratio is also related to the mold draft angle and the plastic material’s stretchability. To ensure product quality, the draw ratio should be controlled to avoid an increase in scrap rate.

3. Fillet Design

Sharp corners should not be designed at the corners or edges of plastic parts. Instead, as large a fillet as possible should be used, with the corner radius generally not smaller than 4 to 5 times the thickness of the sheet. Failure to do so can cause thinning of the material and stress concentration, negatively affecting the part's strength and durability.

4. Draft Angle

Thermoforming molds, similar to regular molds, require a certain draft angle to facilitate demolding. The draft angle typically ranges from 1° to 4°. A smaller draft angle can be used for female molds, as the shrinkage of the plastic part provides some additional clearance, making demolding easier.

5. Reinforcement Rib Design

Thermoformed plastic sheets are usually quite thin, and the forming process is limited by the draw ratio. Therefore, adding reinforcement ribs in structurally weak areas is an essential method for increasing rigidity and strength. The placement of reinforcement ribs should be carefully considered to avoid overly thin areas at the bottom and corners of the part.

In addition, adding shallow grooves, patterns, or markings to the bottom of the thermoformed shell can enhance rigidity and support the structure. Longitudinal shallow grooves on the sides increase vertical rigidity, while transverse shallow grooves, though enhancing resistance to collapse, can make demolding more difficult.

6. Product Shrinkage

Thermoformed products generally experience significant shrinkage, with about 50% of it occurring during cooling in the mold. If the mold temperature is high, the part may shrink by an additional 25% as it cools to room temperature after demolding, with the remaining 25% of shrinkage occurring over the next 24 hours. Moreover, products formed using female molds tend to have a shrinkage rate 25% to 50% higher than those formed with male molds. Therefore, it is crucial to consider shrinkage during the design process to ensure that the final dimensions meet accuracy requirements.

By optimizing the design for geometry, draw ratio, fillet radius, draft angle, reinforcement ribs, and shrinkage, the quality and stability of thermoformed plastic parts can be significantly improved. These process design elements have a crucial impact on the production efficiency and performance of thermoformed products and are key to ensuring that products meet user requirements.